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(BQ) Part 1 book How the immune system works presents the following contents: An overview, the innate immune system, B cells and antibodies, the magic of antigen presentation, T cell activation, T cells at work. How The Immune System Works Lauren Sompayrac th Edition How the Immune System Works I dedicate this book to my sweetheart, my best friend, and my wife: Vicki Sompayrac This title is also available as an e-book For more details, please see www.wiley.com/buy/9781118997772 or scan this QR code: How the Immune System Works Fifth Edition Lauren Sompayrac, PhD This edition first published 2016 © 2016 by John Wiley & Sons, Ltd Previous editions © 1999, 2003, 2008, 2012 John Wiley & Sons, Ltd Registered office: J ohn Wiley & Sons, Ltd, The Atrium, Southern Gate, Chichester, West Sussex, PO19 8SQ, UK Editorial offices: 9600 Garsington Road, Oxford, OX4 2DQ, UK 1606 Golden Aspen Drive, Suites 103 and 104, Ames, Iowa 50010, USA For details of our global editorial offices, for customer services and for information about how to apply for permission to reuse the copyright material in this book please see our website at www wiley.com/wiley-blackwell The right of the author to be identified as the author of this work has been asserted in accordance with the UK Copyright, Designs and Patents Act 1988 All rights reserved No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, except as permitted by the UK Copyright, Designs and Patents Act 1988, without the prior permission of the publisher Designations used by companies to distinguish their products are often claimed as trademarks All brand names and product names used in this book are trade names, service marks, trademarks or registered trademarks of their respective owners The publisher is not associated with any product or vendor mentioned in this book It is sold on the understanding that the publisher is not engaged in rendering professional services If professional advice or other expert assistance is required, the services of a competent professional should be sought The contents of this work are intended to further general scientific research, understanding, and discussion only and are not intended and should not be relied upon as recommending or promoting a specific method, diagnosis, or treatment by health science practitioners for any particular patient The publisher and the author make no representations or warranties with respect to the accuracy or completeness of the contents of this work and specifically disclaim all warranties, including without limitation any implied warranties of fitness for a particular purpose In view of ongoing research, equipment modifications, changes in governmental regulations, and the constant flow of information relating to the use of medicines, equipment, and devices, the reader is urged to review and evaluate the information provided in the package insert or instructions for each medicine, equipment, or device for, among other things, any changes in the instructions or indication of usage and for added warnings and precautions Readers should consult with a specialist where appropriate The fact that an organization or Website is referred to in this work as a citation and/or a potential source of further information does not mean that the author or the publisher endorses the information the organization or Website may provide or recommendations it may make Further, readers should be aware that Internet Websites listed in this work may have changed or disappeared between when this work was written and when it is read No warranty may be created or extended by any promotional statements for this work Neither the publisher nor the author shall be liable for any damages arising herefrom Library of Congress Cataloging-in-Publication Data Sompayrac, Lauren, author   How the immune system works / Lauren Sompayrac Fifth edition        p ; cm   Includes index   ISBN 978-1-118-99777-2 (pbk.)   I Title.    [DNLM: 1.  Immune System physiology 2.  Immune System anatomy & histology 3.  Immune System physiopathology 4.  Immunity physiology.  QW 504]   QR181   616.07’9 dc23 2015015315 A catalogue record for this book is available from the British Library Wiley also publishes its books in a variety of electronic formats Some content that appears in print may not be available in electronic books Cover image and figure on page used with permission from Lennart Nilsson/TT Set in 9.5/13 in Palatino LT Std by Aptara, India Printed in [Country only] 1 2016 Contents Acknowledgments, vii How to Use This Book, viii This book is neither a comprehensive text nor an exam-review tool It is an overview of the immune system, designed to give anyone who is learning immunology a feel for how the system fits together Lecture An Overview, The immune system is a “team effort,” involving many different players who work together to provide a powerful defense against invaders Focusing in on one player at a time makes it hard to understand the game Here we view the action from the grandstands to get a wide-angle picture of what the immune system is all about Lecture The Innate Immune System, 13 The innate immune system is a “hard-wired” defense that has evolved over millions of years to recognize pathogens that commonly infect humans It provides a rapid and powerful response against “everyday” invaders Lecture B Cells and Antibodies, 27 B cells and the antibodies they produce are part of the adaptive immune system This defense evolves during our own lifetime to protect us against invaders that we, personally, have never encountered before Lecture The Magic of Antigen Presentation, 42 T cells, another weapon of the adaptive immune system, only recognize invaders which are “properly presented” by specialized antigen presenting cells This feature keeps these important cells focused on the particular attackers which they are able to defend against Lecture T Cell Activation, 55 Before they can spring into action, T cells must be activated This requirement helps insure that only useful weapons will be mobilized Lecture T Cells at Work, 63 Once they have been activated, helper T cells orchestrate the immune response, and killer T cells destroy infected cells v vi Contents Lecture Secondary Lymphoid Organs and Lymphocyte Trafficking, 72 B and T lymphocytes travel through secondary lymphoid organs looking for the intruders they can defend against Once activated in the secondary lymphoid organs, B and T cells are dispatched to the particular areas of the body where they can be most useful Lecture Restraining the Immune System, 84 The powerful weapons of the immune system must be restrained lest they become overexuberant In addition, once an invader has been defeated, the immune system must be “reset” to prepare for future attacks Lecture Self Tolerance and MHC Restriction, 88 T cells must be trained to focus on appropriately presented invaders, and B and T lymphocytes must learn not to attack our own bodies Lecture 10 Immunological Memory, 98 The innate immune system remembers pathogens which have been attacking humans for millions of years In contrast, B and T cells remember pathogens we have encountered during our lifetime Memory B and T lymphocytes respond more quickly and effectively to a subsequent attack by the same invader Lecture 11 The Intestinal Immune System, 103 The human intestines are home to trillions of bacteria, viruses, fungi, and parasites How the immune system deals with these potentially dangerous intestinal residents, which frequently invade the tissues surrounding the intestines, is a hot topic in immunology Lecture 12 Vaccines, 110 Vaccines safely mimic the attack of an invader so that our immune system will be primed and ready for a future challenge by the same invader Lecture 13 The Immune System Gone Wrong, 116 The immune system generally does a good job of defending us without causing a lot of “collateral damage.” Sometimes, however, mistakes are made Lecture 14 Immunodeficiency, 126 Serious disease may result when our immune system does not operate at full strength Humans who are infected with the AIDS virus have profoundly impaired immune systems Lecture 15 Cancer and the Immune System, 131 The human immune system is not very good at defending us against cancer Indeed, there is a built-in conflict between the need to minimize the chance that its weapons will attack our own bodies, and the need to destroy wannabe cancer cells Glossary, 139 Here are definitions of some of the terms that immunologists use – but which “normal” people wouldn’t List of Acronyms and Abbreviations, 142 Immunologists are big on acronyms and abbreviations, so I’ve made a list to which you can refer Index, 143 Acknowledgments I would like to thank the following people, whose critical comments on earlier editions were most helpful: Drs Mark Dubin, Linda Clayton, Dan Tenen, Jim Cook, Tom Mitchell, Lanny Rosenwasser, and Eric Martz Thanks also go to Diane Lorenz, who illustrated the first and second editions, and whose wonderful artwork still can be found in this book Finally, I wish to thank Vicki Sompayrac, whose wise suggestions helped make this book more readable, and whose editing was invaluable in preparing the final manuscript vii How to Use This Book I wrote How the Immune System Works because I couldn’t find a book that would give my students an overall view of the immune system Sure, there are as many good, thick textbooks as a person might have money to buy, but these are crammed with every possible detail There are also lots of “review books” that are great if you want a summary of what you’ve already learned  –  but they won’t teach you immunology What was missing was a short book that tells, in simple language, how the immune system fits together – a book that presents the big picture of the immune system, without the jargon and the details How the Immune System Works is written in the form of “lectures,” because I want to talk to you directly, just as if we were together in a classroom Although Lecture is a light-hearted overview, meant to give you a running start at the subject, you’ll soon discover that this is not “baby immunology.” How the Immune System Works is a concept-driven analysis of how the immune system players work together to protect us from disease – and, most importantly, why they it this way In Lectures through 10, I focus more closely on the individual players and their roles These lectures are short, so you probably can read them all in a couple of afternoons In fact, I strongly suggest that you begin by reading quickly through Lectures 1–10 The whole idea is to get an overall view of the subject, and if you read one lecture a week, that won’t happen Don’t “study” these 10 lectures your first time through Don’t even bother with the Thought Questions at the end of each lecture Just rip through them Then, once you have a “feel” for viii the system, go back and spend a bit more time with these same 10 lectures to get a clearer understanding of the “hows and whys.” In Lectures 11–15, I discuss the intestinal immune system, vaccines, allergies, autoimmune disease, the AIDS virus, and cancer These lectures will let you “practice” what you have learned in the earlier lectures by examining real-world examples of the immune system at work So after you have gone through Lectures 1–10 twice, I’d suggest you read these last five lectures When you do, I think you’ll be amazed by how much you now understand about the immune system As you read, you will encounter passages highlighted in blue, and words that are highlighted in red These highlights are to alert you to important concepts and terms They also will help you review a lecture quickly, once you have read it through In some settings, How the Immune System Works will serve as the main text for the immunology section of a larger course For a semester-long undergraduate or graduate immunology course, your professor may use this book as a companion to a comprehensive textbook As your course proceeds, reviewing the appropriate lectures in How the Immune System Works will help you keep the big picture in focus as the details are filled in It’s really easy to get lost in the details No matter how your professor may choose to use this book, you should keep one important point in mind: I didn’t write How the Immune System Works for your professor This book is for you! L ECTU R E   T Cell Activation    57 α β Outside Cell Cell Membrane ε δ Inside Cell ε γ ζ ζ To handle the signaling chores, Mother Nature had to add a few bells and whistles to the TCR: a complex of proteins collectively called CD3 In humans, this signaling complex is made up of four different proteins: γ, δ, ε, and ζ (gamma, delta, epsilon, and zeta) The CD3 proteins are anchored in the cell membrane, and have cytoplasmic tails that are long enough to signal just fine Please note, however, that the γ and δ proteins that are part of the CD3 complex are not the same as the γ and δ proteins that make up the γδ T cell receptor The whole complex of proteins (α, β, γ, δ, ε, ζ) is transported to the cell surface as a unit If any one of these proteins fails to be made, you don’t get a TCR on the surface Consequently, most immunologists consider the functional, mature TCR to be this whole complex of proteins After all, the α and β proteins are great for recognition, but they can’t signal And together, the γ, δ, ε, and ζ proteins signal just fine, but they are totally blind to what’s going on outside the cell You need both parts to make it work As with BCRs, signaling involves clustering TCRs together in one area of the T cell surface When this happens, a threshold number of kinase enzymes are recruited by the cytoplasmic tails of the CD3 proteins, and the activation signal is dispatched to the nucleus When the α and β chains of the TCR were first discovered, it was thought that the TCR was just an on/ off switch whose only function was to signal activation But now that you have heard about the CD3 proteins, let me ask you: Does this look like a simple on/off switch? No way! Mother Nature certainly wouldn’t make an on/off switch with six proteins No, this TCR is quite versatile It can send signals that result in very different outcomes, depending on how, when, and where it is triggered For example, in the thymus, if a T cell’s receptors recognize MHC plus self peptide, the TCRs trigger the T cell to commit suicide to prevent autoimmunity Later in its life, if its TCRs recognize their cognate antigen presented by MHC molecules, but that T cell does not receive the required co‐stimulatory signals, the T cell may be neutered (anergized) so it can’t function And, of course, when a TCR is presented with its cognate antigen, and co‐stimulatory signals are available, the TCR can signal activation So this same T cell receptor, depending on the situation, signals death, anergy, or activation In fact, there are now documented cases in which the alteration of a single amino acid in a presented peptide can change the signal from activation to death! Clearly this is no on/off switch, and immunologists are working very hard to understand exactly how TCR signaling is “wired,” and what factors influence the signaling outcomes CD4 AND CD8 CO‐RECEPTORS In addition to the T cell receptor, there are two more molecules which are involved in antigen recognition by T cells  –  the CD4 and CD8 co‐receptors Now, doesn’t it seem that Mother Nature got carried away when she added on these CD4 and CD8 co‐receptors? I mean, there are already two proteins, α and β, to use for antigen recognition, and four more, γ, δ, ε, and ζ, to use for signaling Wouldn’t you think that would it? Apparently not, so there must be essential features of the system that require CD4 and CD8 co‐receptors Let’s see what these might be Killer T cells and helper T cells perform two very different functions, and they “look at” two different molecules, class I or class II MHC, respectively, to get their cues But how CTLs know to focus on peptides presented by class I molecules – and how Th cells know to scan APCs for peptides presented by class II? After all, it wouldn’t be so great if a CTL got confused, recognized a class II–peptide complex on an APC, and killed that antigen presenting cell So here’s where CD4 and CD8 come in CTLs generally express CD8, and Th cells usually express CD4 These co‐receptor molecules are designed to clip onto either class I MHC (CD8) or class II MHC molecules (CD4) 58  LECT URE 5  T Cell Activation CO‐STIMULATION APC APC MHC I MHC II peptide peptide CD4 CD8 TCR CD3 CTL TCR CD3 Th Cell These “clips” strengthen the adhesion between the T cell and the APC somewhat, so CD4 and CD8 co‐ receptors help focus the attention of Th cells and CTLs on the proper MHC molecule But there is more to the story It turns out that CD4 and CD8 are signaling molecules just like the CD3 complex of proteins Both CD4 and CD8 have tails that extend through the cell’s plasma membrane and into the cytoplasm, and both of these tails have the right characteristics to signal In contrast to CD3 molecules, which are glued rather tightly to the αβ T cell receptor on the cell surface, the CD4 and CD8 co‐receptors usually are only loosely associated with the TCR/CD3 proteins The idea is that after a TCR has engaged its cognate antigen presented by an MHC molecule, the CD4 or CD8 co‐receptors then clip on, help stabilize the TCR– MHC–peptide interaction, and strengthen the signal sent by the TCR When T cells begin maturing in the thymus, they express both types of co‐receptors on their surface Immunologists call them CD4+CD8+ or “double‐positive” cells Then, as they mature, expression of one or the other of these co‐receptors is downregulated, and a cell becomes either CD4+ or CD8+ So how does a given T cell decide whether it will express the CD4 or the CD8 co‐receptor when it grows up? Although there is still some disagreement about this process, the latest thinking is that the type of MHC molecule the TCR recognizes determines this choice If a T cell’s receptors have the right structure to recognize a class I MHC molecule, that T cell downregulates expression of CD4, and only displays the CD8 co‐receptor molecule on its surface On the other hand, if a T cell’s receptors prefer to bind to a class II MHC molecule, that T cell is “instructed” to continue to express the CD4 but not the CD8 co‐receptor In naive T cells, the “connection” between the T cell’s receptors and the cell’s nucleus is not very good It’s as if the T cell had an electrical system in which a large resistor were placed between the sensor (the TCR) and the piece of equipment it is designed to regulate (gene expression in the nucleus) Because of this “resistor,” a lot of the signal from the TCR is lost as it travels to the nucleus The result is that a prohibitively large number of TCRs would have to engage their cognate antigen before the signal that reaches the nucleus would be strong enough to have any effect If, however, while the TCRs are engaged, the T cell also receives co‐stimulation, the signal from the TCRs is amplified many times, so that fewer (probably about 100‐fold fewer) TCRs must be engaged to activate a naive T cell Although a number of different molecules have been identified which can co‐stimulate T cells, certainly the best studied examples are the B7 proteins (B7‐1 and B7‐2) which are expressed on the surface of antigen presenting cells B7 molecules provide co‐stimulation to T cells by plugging into receptor molecules called CD28 on the T cell’s surface So in addition to having their T cell receptors ligated by MHC–peptide, naive T cells also must receive co‐ stimulatory signals before they can be activated Co‐ stimulation can be thought of as an “amplifier” that strengthens the “I’m engaged” signal sent by a T cell’s receptors, thereby lowering the threshold number of TCRs which must be crosslinked by MHC–peptide complexes Interestingly, once a naive T cell has been activated, the connection between the TCRs and the nucleus strengthens It is as if an experienced T cell has been “re‐wired” so that the resistor present in a naive T cell is bypassed As a result of this re‐wiring, in an experienced T cell, amplification of the TCR signal is not as important as it is in a virgin T cell Consequently, experienced T cells have a reduced requirement for co‐stimulation A TIME‐LAPSE PHOTO OF HELPER T CELL ACTIVATION In the lymph nodes, helper T cells quickly scan dendritic cells to see if their cognate antigen is being displayed A single dendritic cell typically hosts about 1000 such “visits” each hour If a T cell does find a dendritic L ECTU R E   T Cell Activation    59 cell displaying its cognate antigen, the T cell “lingers,” because complete activation of a naive helper T cell usually takes several hours During this time, a number of important events take place First, adhesion molecules on the surface of the dendritic cell bind to their adhesion partners on the T cell, helping keep the two cells together Next, the CD4 co‐receptor molecules on the surface of the T cell clip onto the class II MHC molecules on the dendritic cell and strengthen the interaction between the two cells In addition, the engagement of its TCRs upregulates the expression of adhesion molecules on the Th cell surface, strengthening the “glue” that holds the APC and the T cell together This is important, because the initial binding between a TCR and an MHC–peptide complex is actually rather weak to allow for rapid scanning Consequently, the Velcro‐ like adhesion molecules are extremely important for T cell activation The clustering of TCRs and adhesion molecules at the point of contact between an APC and a T cell results in the formation of what immunologists call an immunological synapse Engagement of a helper T cell’s receptors also upregulates expression of CD40L proteins on its surface, and when these proteins plug into the CD40 proteins on the surface of a dendritic cell, several remarkable things happen Although mature dendritic cells express MHC and co‐stimulatory molecules (e.g., B7) when they first enter lymph nodes, the expression level of these proteins increases when CD40 proteins on the APC are engaged by the CD40L proteins on a Th cell In addition, engagement of a dendritic cell’s CD40 proteins prolongs the life of the dendritic cell This extension of a “useful” dendritic cell’s life span makes perfect sense It insures that the particular dendritic cells which are presenting a T cell’s cognate antigen will stick around long enough to help activate a lot of these T cells So the interaction between a dendritic cell and a naive helper T cell is not just one‐way These cells actually perform an activation “dance” in which they stimulate each other The end result of this cooperation is that the dendritic cell becomes a more potent antigen presenting cell, and the Th cell is activated to express the high levels of CD40L required for helping activate B cells After activation is complete, the helper T cell and the antigen presenting cell part The APC then goes on to activate other T cells, while the recently activated Th cells proliferate to build up their numbers During an infection, a single activated T cell can give rise to about 10 000 daughter cells during the first week or so of proliferation This proliferation is driven by growth factors such as IL‐2 Naive T cells can make some IL‐2, but they don’t have IL‐2 receptors on their surface – so they can’t respond to this cytokine In contrast, activated Th cells produce large amounts of IL‐2, and they also express receptors for this cytokine on their surface As a result, newly activated helper T cells stimulate their own proliferation This coupling of activation to the upregulation of growth factor receptors is the essence of clonal selection: Those Th cells which are selected for activation (because their TCRs recognize an invader) upregulate their growth factor receptors, and proliferate to form a clone So the sequence of events during the activation of a helper T cell is this: Adhesion molecules mediate weak binding between the Th and the APC while TCRs engage their cognate antigen presented by the APC Receptor engagement strengthens the adhesion between the two cells, and upregulates CD40L expression on the Th cell CD40L then binds to CD40 on the APC and stimulates expression of MHC and co‐stimulatory molecules on the APC surface The co‐stimulation provided by the APC amplifies the “TCR engaged” signal, making activation of the Th cell more efficient When activation is complete, the cells disengage, and the Th cell proliferates, driven by growth factors which bind to receptors that appear on the Th cell surface as a result of activation This proliferation produces a clone of helper T cells which can recognize the invader advertised by the antigen presenting cell HOW KILLER T CELLS ARE ACTIVATED For a helper T cell to be activated, its receptors must recognize their cognate antigen displayed by class II MHC molecules on the surface of an activated dendritic cell, and the Th cell must also receive co‐stimulatory signals from that same dendritic cell This requirement that two cells (the Th cell and the DC) agree that there has been an invasion is a powerful safeguard against the activation of “rogue” helper T cells – cells which might direct an attack against our own tissues, causing autoimmune disease Although the events involved in the activation of helper T cells are pretty clear, the picture of how naive killer T cells are activated is still rather fuzzy Until recently, it was believed that for a naive killer T cell to 60  LECT URE 5  T Cell Activation be activated, three cells needed to be involved: a CTL with receptors that recognized the invader; an activated dendritic cell, which was using its class I MHC molecules to present fragments of the invader’s proteins to the CTL; and an activated helper T cell which was providing “help” to the CTL One way this might happen would be for the dendritic cell, the Th cell, and the CTL to engage in a ménage trois There is, however, a potential problem with this scenario Early in an infection, there are very few of any of these cells around Consequently, the probability is quite small that a helper T cell and a killer T cell would simultaneously find a dendritic cell which is presenting their cognate antigens Experiments have now shown that, in response to an invasion by microbes which can infect cells (the microbes that CTLs are designed to defend against), T cell help is not required during the initial activation of killer T cells A two‐cell interaction between a naive CTL and an activated dendritic cell is sufficient During this meeting, the CTL’s receptors recognize their cognate antigen displayed by class I MHC molecules on the dendritic cell, and they receive the co‐stimulation they need from that same dendritic cell What this means is that a naive killer T cell can be activated in a way that is analogous to the way a naive Th cell is activated: by encountering an activated dendritic cell Requiring only a two‐cell interaction for the activation of naive Th cells and CTLs makes perfect sense in terms of getting the adaptive immune system fired up before invaders take over completely However, activation of naive killer T cells without Th cell help does raise an important question: If Th cells are supposed to be orchestrating the immune response, just what is their contribution in terms of giving directions to killer T cells? Although a single CTL is capable of killing many target cells sequentially, thousands of cells usually are infected during an attack Consequently, many CTLs are required to repulse a serious attack Killer T cells require IL‐2 for continued proliferation, and activated helper T cells are the major supplier of this growth factor If there is a dangerous invasion, many Th cells will be activated These cells will produce a lot of IL‐2, which will encourage massive proliferation of activated CTLs So by supplying a killer T cell’s favorite growth factor, IL‐2, Th cells can help control the strength of the CTL response Experiments also have shown that when CTLs are activated without Th cell help, they proliferate somewhat to build up their numbers and they can kill infected cells Nevertheless, these “helpless” CTLs not kill with high efficiency, and they not live very long It is as if helpless activation of CTLs results in a small “burst” of killer T cells designed to deal quickly with invaders early in an infection In contrast, in order to efficiently activate long‐lived killer T cells or to generate memory killer T cells (cells which can defend against a subsequent invasion by the same attacker) assistance from helper T cells is required And this, of course, brings us back to the question of how CTLs can be fully activated by Th cells and DCs without requiring a three‐cell interaction One possibility, the “sequential model,” postulates that when helper T cells are activated, the dendritic cells which activate them become “licensed” to activate CTLs – thus avoiding the need for all three cells to meet simultaneously It has also been demonstrated that when an activated dendritic cell and a helper T cell “hook up,” chemokines are generated which can attract naive killer T cells to their location, making a ménage trois more likely Moreover, the probability of such a three‐cell interaction also is increased by the fact that the meeting between an activated dendritic cell and a helper T cell typically lasts for several hours Consequently, cytokine‐ directed migration and extended Th–APC interaction times could give that rare CTL which also recognizes the invader a better chance to join the party Finally, relatively late in an immune response, there will be many activated dendritic cells, Th cells, and killer T cells present in lymph nodes and other secondary lymphoid organs  –  perhaps enough of each of these cell types to make a three‐cell interaction probable My guess is that killer T cells can be activated by several of these mechanisms But stay tuned! FAIL‐SAFE ACTIVATION For either a naive helper T cell or a virgin CTL to be activated, the T cell must first recognize its cognate antigen presented by an antigen presenting cell This is true even for helper T cell‐independent activation of CTLs As a consequence of this requirement for antigen presentation during activation, a fail‐safe system is set up in which the decision to activate a T cell always involves more than one cell This helps insure that the powerful weapons of the adaptive immune system only come into play when there is a genuine threat, and makes it less likely that a T cell will turn its weapons inward on its host L ECTU R E   T Cell Activation    61 REVIEW There are many similarities between the ways B cells and T cells are activated BCRs and TCRs both have “recognition” proteins that extend outside the cell, and which are incredibly diverse because they are made by mixing and matching gene segments For the BCR, these recognition proteins are the light and heavy chains that make up the antibody molecule For the TCR, the molecules that recognize antigen are the α and β proteins TCRs and BCRs have cytoplasmic tails that are too short to signal recognition, so additional molecules are required for this purpose For the BCR, these signaling proteins are called Igα and Igβ For the TCR, signaling involves a complex of proteins called CD3 For B and T cells to be activated, their receptors must be clustered by antigen, because this crosslinking brings together many of their signaling molecules in a small region of the cell When the density of signaling molecules is great enough, an enzymatic chain reaction is set off that conveys the “receptor engaged” signal to the cell’s nucleus There, in the “brain center” of the cell, genes involved in activation are turned off or on as a result of this signal Although crosslinking of receptors is essential for the activation of B and T cells, it is not enough Naive B and T cells also require co‐stimulatory signals that are not antigen specific This two‐signal requirement for activation sets up a fail‐safe system which protects against the inappropriate activation of B and T cells For B cell activation, a helper T cell can provide co‐stimulation through surface proteins called CD40L that plug into CD40 proteins on the B cell surface B cells also can be co‐stimulated by “danger signals,” including invader‐ specific molecular signatures or battle cytokines For T cells, co‐stimulation usually involves B7 proteins on an activated dendritic cell that engage CD28 proteins on the surface of the T cell Both BCRs and TCRs can associate with co‐receptor molecules which serve to amplify the signal that the BCRs and TCRs send For B cells, this co‐receptor recognizes antigen that has been opsonized by complement If the BCR recognizes an antigen, and if that antigen also is “decorated” with complement protein fragments, the antigen serves as a “clamp” that brings the BCR and the complement receptor together on the surface of the B cell, greatly amplifying the “receptor engaged” signal As a consequence, B cells are much more easily activated (many fewer BCRs need to be crosslinked) by antigen that has been opsonized by complement T cells also have co‐receptors Th cells express CD4 co‐receptor molecules on their surface, and CTLs express CD8 co‐receptors When a TCR binds to antigen presented by an MHC molecule, the co‐receptor on the T cell surface clips onto the MHC molecule This serves to strengthen the signal that is sent by the TCR to the nucleus, so that the T cell is more easily activated (fewer TCRs need to be crosslinked) These co‐receptors only work with the “right” MHC types: class I for CTLs with CD8 co‐receptors, and class II for Th cells with CD4 co‐ receptors Consequently, co‐receptors really are “focus” molecules The B cell co‐receptor helps B cells focus on antigens that have already been identified by the complement system as dangerous (those that have been opsonized) The CD4 co‐receptor focuses the attention of Th cells on antigens displayed by class II MHC molecules, and the CD8 co‐receptor focuses CTLs on antigens displayed by class I MHC molecules Of course, there is an important difference between what B cells and T cells “look at.” The BCR recognizes antigen in its “natural” state – that is, antigen which has not been chopped up and bound to MHC molecules This antigen can be a protein or almost any other organic molecule (e.g., a carbohydrate or a fat) In contrast, the αβ receptors of traditional T cells only recognize fragments of proteins presented by MHC molecules And whereas a B cell’s receptors only bind to one thing – its cognate antigen – the TCR binds to both the presented peptide and the MHC molecule Because the universe of antigens recognized by the BCR includes proteins, carbohydrates, and fats, B cells can respond to a greater variety of invaders than can T cells On the other hand, because the TCR looks at small fragments of proteins, it can recognize targets that are hidden from view of the BCR in an intact and tightly folded protein Another difference between B cells and T cells is that during an infection, the BCR can undergo somatic hypermutation and selection So B cells can “draw from the deck” to try to get a better hand In contrast, the TCR does not hypermutate, so T cells must be satisfied with the cards they are dealt 62  LECT URE 5  T Cell Activation THOUGHT QUESTIONS What is the difference between a co‐receptor and co‐stimulation? Give examples and tell why each is important for B or T cell activation Why are cellular adhesion molecules important during T cell activation? Don’t these “sticky” molecules just slow the process down? What happens when dendritic cells and helper T cells “dance”? Essentially all players on the innate and adaptive immune system teams must be activated before they can “get into the game.” Trace the steps in the “activation cascade” which begins when an LPS‐carrying, Gram‐negative bacterium enters a wound, and which ends when antibodies are produced that can recognize the bacterium Mother Nature uses “fail‐safe technology” to prevent inappropriate activation of the immune system Can you give several examples? LECTURE T Cells at Work HEADS UP! The innate immune system “instructs” the adaptive immune system, telling it which weapons to mobilize to defend against a given invader, and where these weapons should be deployed in the body Two of the most important weapons are helper T cells, which secrete just the right combination of cytokines to orchestrate an appropriate defense, and killer T cells, which can “execute” infected cells and the pathogens within them INTRODUCTION Once helper T cells and killer T cells have been activated, they are ready to go to work – to become what immunologists call effector cells The primary job of an effector CTL is to kill cells that have been infected by viruses or bacteria Effector helper T cells have two main duties First, they can remain in the blood and lymphatic circulation and travel from node to node, providing help for B cells or for killer T cells In addition, effector helper T cells can exit blood vessels at the sites where a battle is going on to provide help for the soldiers of the innate and adaptive immune systems HELPER T CELLS AS CYTOKINE FACTORIES Helper T cells can produce many different cytokines –  protein molecules which they use to communicate with the rest of the immune system As the “quarterback” of the immune system team, the helper T cell uses cytokines to “call the plays.” These include cytokines such as TNF, IFN‐γ, IL‐4, IL‐5, IL‐6, IL‐10, IL‐17, and IL‐21 However, a single Th cell doesn’t secrete all these different cytokines In fact, Th cells tend to secrete subsets of cytokines  –  subsets which are appropriate to orchestrate an immune defense against particular invaders So far, three major subsets have been identified: Th1, Th2, and Th17 You shouldn’t take this to mean, however, that there are only three different combinations of cytokines that can be secreted by Th cells In fact, immunologists initially had a hard time finding helper T cells that secreted exactly the Th1 or Th2 cytokine subsets in humans Clearly, there are helper T cells which give off mixtures of cytokines that don’t conform to the Th1/Th2/ Th17 paradigm Nevertheless, this concept turns out to be quite useful in trying to make sense of the combination of cytokines (the cytokine “profile”) that Th cells produce I also should mention that in addition to these three Th subsets which are involved in activating the immune system, there is a subset of Th cells which functions to suppress the immune response We will discuss those “Treg” cells in subsequent lectures Of course, all of this begs the question: How does a helper T cell know which cytokines are appropriate for a given situation? Well, as any football fan knows, behind every good quarterback, there is a good coach THE DENDRITIC CELL AS “COACH” OF THE IMMUNE SYSTEM TEAM For a helper T cell to make an informed decision about which cytokines to make, at least two pieces of information are required First, it’s necessary to know what type of invader the immune system is dealing with Is it a virus, a bacterium, a parasite, or a fungus? Second, it is essential to determine where in the body the invaders How the Immune System Works, Fifth Edition Lauren Sompayrac © 2016 John Wiley & Sons, Ltd Published 2016 by John Wiley & Sons, Ltd 63 64  LECT URE 6  T Cells at Work are located Are they in the respiratory tract, the digestive tract, or the big toe? Virgin helper T cells don’t have direct access to either type of information After all, they are busy circulating through the blood and lymph, trying to find their cognate antigen What is needed is an “observer” who actually has been at the battle site, who has collected the pertinent information, and who can pass it along to the helper T cell And which of the immune system cells could qualify as such an observer? The dendritic antigen presenting cell, of course! Just as a football coach scouts the opposing team and formulates a game plan, so a dendritic cell, acting as the “coach” of the immune system team, collects information on the invasion, and decides how the immune system should react That’s why dendritic cells are so important They don’t just turn naive helper T cells and killer T cells on Dendritic cells actually function as the “brains” of the immune system, processing the information pertaining to the invasion, and producing a plan of action What are the inputs that dendritic cells integrate to produce the game plan? These are of two types The first input comes to the dendritic cell through the pattern‐recognition receptors we discussed in Lecture These cellular receptors recognize conserved patterns that are characteristic of various classes of invaders For example, Toll‐like receptor (TLR4) senses the presence of LPS, which is a molecule that is a component of the outer cell membrane of Gram‐negative bacteria TLR4 also can detect proteins made by certain viruses TLR2 specializes in identifying proteins that are “signatures” of Gram‐positive bacteria TLR3 recognizes the double‐stranded RNA produced during many viral infections And TLR9 recognizes the unmethylated DNA dinucleotide, CpG, which is characteristic of bacterial DNA Although TLRs were the first pattern‐recognition receptors to be characterized, additional families of pattern‐recognition receptors have now been discovered Consequently, the emerging picture is that different types of antigen presenting cells (e.g., dendritic cells or macrophages) in different parts of the body display distinct sets of these pattern‐recognition receptors which are “tuned” to recognize various structural features of common microbial invaders By integrating the signals from these diverse pattern‐recognition receptors, an APC gathers information on the type of invader to be defended against The second “scouting report” dendritic cells employ when formulating their game plan is received through various cytokine receptors on their surface Because different pathogens elicit the production of different cytokines during an infection, dendritic cells can learn a lot about an invader by sensing the cytokine environment So dendritic cells out on the front lines collect “intelligence” about an invader through pattern‐recognition receptors and cytokine receptors It is then up to the dendritic cell to “decode” these inputs in order to discern the type of invasion, and to decide which weapons need to be mobilized Cells in different areas of the body (e.g., skin cells or cells that underlie the intestines) produce characteristic mixtures of cytokines in response to invaders, and these cytokines provide dendritic cells with information about the area of the body that is under attack In fact, these cytokines imprint dendritic cells with a “regional identity.” And this ability to remember their “roots” helps DCs dispatch the weapons of the adaptive immune system to the parts of the body where they are needed But how is the dendritic cell’s game plan conveyed to the Th cell – the cell that will direct the action? There are two ways that the coach instructs the quarterback First, the mixture of co‐stimulatory molecules displayed on the surface of an activated dendritic cell will depend on the type of invader the DC has encountered These co‐stimulatory molecules can “plug into” receptor molecules on the surface of helper T cells to pass this information along Although B7 is the best‐studied co‐stimulatory molecule, other co‐stimulatory molecules have been identified, and more are certain to be discovered In addition to co‐stimulatory surface molecules, activated dendritic cells produce cytokines which also can convey information to the helper T cell So the bottom line is this: Co‐stimulatory molecules and cytokines are used by dendritic cells to pass along the “game plan” to helper T cells And the particular combination of co‐ stimulatory molecules and cytokines which a dendritic cell offers to a Th cell will depend on what the dendritic cell has observed at the battle scene To get a better idea of how this all works, let’s look more closely at the Th1, Th2, and Th17 subsets of cytokines Th1 HELPER T CELLS If you have a puncture wound that results in a bacterial infection or if you are attacked by a virus that replicates in the tissues, resident dendritic cells will be alerted through their pattern‐recognition receptors and L ECTU R E   T Cells at Work    65 by receiving battle cytokines produced by macrophages and other cells in the inflamed tissues These signals activate the dendritic cell and imprint it with the special characteristics of an APC which has observed a bacterial or viral infection in the tissues The details of exactly how this is accomplished aren’t clear yet, but the result is that when this DC leaves such a battle site and travels through the lymph to a nearby lymph node, it will produce the cytokine IL‐12 And when the IL‐12‐producing DC presents the battle antigens it has acquired to a virgin helper T cell, that Th cell will be instructed to become a helper T cell which produces the “classical” Th1 cytokines: TNF, IFN‐γ, and IL‐2 Th2 HELPER T CELLS Now suppose that you have been infected by a parasite (e.g., hook worms) or you have eaten some food that is contaminated with pathogenic bacteria In the tissues that line your intestines, a battle will be raging Dendritic cells from that area will travel to nearby lymph nodes, and will activate those helper T cells which have T cell receptors that can recognize the worm or bacterial antigens presented by the DC This interaction results in helper T cells which are “programmed” to produce the Th2 subset of cytokines, which includes IL‐4, IL‐5, and IL‐13 IL-4 IL-12 Antigen Antigen Th1 Cell DC IL-4 TNF IFN-γ Th2 Cell DC IL-2 TCR TCR Class II MHC Molecule Class II MHC Molecule Why these particular cytokines? Let’s see what these cytokines The TNF secreted by Th1 helper T cells helps activate macrophages and natural killer cells However, macrophages only stay activated for a limited time They are lazy fellows which like to go back to resting and garbage collecting Fortunately, the IFN‐γ produced by Th1 cells acts as a “prod” that keeps macrophages fired up and engaged in the battle IFN‐γ also influences B cells during class switching to produce human IgG3 antibodies These antibodies are especially good at opsonizing viruses and bacteria and at fixing complement NK cells can kill three or four target cells in about 16 hours, but then they “tire out.” The IL‐2 produced by Th1 cells can “recharge” NK cells, enabling them to kill some more In addition, IL‐2 is a growth factor which stimulates the proliferation of CTLs, NK cells, and Th1 cells themselves – so that more of these important weapons will be available to deal with the attack Altogether, the Th1 cytokines are the perfect package to help defend against a viral or bacterial attack in the tissues The Th1 cytokines instruct the innate and adaptive systems to mobilize cells and produce antibodies that are especially effective against these invaders, and these cytokines also keep the warriors of the immune system fired up until the invaders have been defeated IL-5 IL-13 Why IL‐4, IL‐5, and IL‐13, you ask? IL‐4 is a growth factor that stimulates the proliferation of helper T cells which have committed to secrete the Th2 profile of cytokines So, like Th1 cells, Th2 cells produce their own growth factor IL‐4 also is a growth factor for B cells, and this cytokine can influence B cells to class switch to produce IgE antibodies  –  powerful weapons against parasites such as hook worms IL‐5 is a cytokine which encourages B cells to produce IgA antibodies, antibodies that are especially useful against bacteria which invade via the digestive tract And IL‐13 stimulates the production of mucus in the intestines This helps prevent more intestinal parasites or pathogenic bacteria from breaching the intestinal barrier and entering the tissues So the Th2 cytokine profile is just the ticket if you need to defend against parasites or pathogenic bacteria that have invaded via the digestive tract In the figure above, you will notice that IL‐4, which causes a naive Th cell to commit to becoming a Th2 cell, does not come from the dendritic cell Of course, once the helper T cell commits to the Th2 cytokine profile, there will be plenty of IL‐4 around – because this is one of the cytokines Th2 cells secrete However, the initial source of IL‐4 required for Th2 commitment has not yet been identified 66  LECT URE 6  T Cells at Work Th17 HELPER T CELLS The existence of helper T cells which produce the Th17 cytokine profile is a recent discovery, and less is known about Th17 cells than about Th1 and Th2 helper T cells One reason for this is that Th17 cells function, at least in part, in the defense against fungi  –  and the immune system’s response to a fungal attack is not nearly so well researched as the immune defense against bacteria or viruses Consequently, the story on Th17 cells is still incomplete, but here’s the emerging picture If a dendritic cell is stationed in an area of the body which is being attacked by fungi (e.g., a vaginal yeast infection) or by certain extracellular bacteria, that DC will travel to a nearby lymph node to activate helper T cells which recognize the antigens the DC is presenting These traveling dendritic cells will produce TGFβ and IL‐6, which together with co‐stimulatory molecules, influence newly activated helper T cells to produce the Th17 subset of cytokines, which includes IL‐17 and IL‐21 TGFβ IL-6 complement system So if you are attacked by fungi or extracellular bacteria, the cytokines secreted by Th17 cells are there to help protect you Th0 HELPER T CELLS Some helper T cells (the Th0 cells) remain “unbiased” when they first are activated, retaining the ability to produce a wide range of cytokines It appears that DCs tell these helper T cells where to go, but not what to However, once Th0 cells reach the battle scene, the cytokine environment they encounter there causes them to commit to the cytokine profile required for the defense For example, when Th0 cells exit the blood to fight a bacterial infection in the tissues, they encounter an environment rich in IL‐12 This is because Th1 cells that are already fighting bacteria there produce IFN‐γ This cytokine, together with danger signals like the bacterial molecule LPS, activates tissue macrophages, which secrete large amounts of IL‐12 And when Th0 cells receive the IL‐12 signal, they “realize” what type of battle is being fought, and commit to becoming Th1 cells – which produce the cytokines needed to defend against bacteria Antigen IFN-γ IL-17 Th17 Cell DC LPS Th1 IL-21 Class II MHC Molecule -γ " TCR MΦ " m a ke re mo IF N IL-12 Th0 IL‐21 encourages uncommitted Th cells to become Th17 cells, and this increases the number of Th17 cells available to battle the fungus Secretion of the “signature cytokine,” IL‐17, results in the recruitment of massive numbers of neutrophils to the site of infection These neutrophils help defend against pathogens against which Th1 and Th2 cells are relatively ineffective, including fungi and some extracellular bacteria – bacteria which not enter cells Indeed, patients who have a genetic defect in IL‐17 secretion suffer from devastating fungal infections (e.g., infection with the common yeast, Candida albicans) even though their Th1 and Th2 helper T cells function normally IL‐17 and IL‐21 influence B cells to produce antibody classes that can opsonize fungi or bacteria and can activate the Th1 " b e co m e a T h " IL-12 Likewise, Th0 cells can become Th2 or Th17 cells when they reach a battle site that is rich in IL‐4 or IL‐6 and TGFβ, respectively So previously uncommitted Th0 cells can be “converted” by the cytokine environment at the scene of the battle to become Th1, Th2, or Th17 cells LOCKING IN THE HELPER T CELL PROFILE Once helper T cells commit to a particular cytokine profile, they begin to secrete cytokines which encourage the proliferation of that particular type of Th cell – be it Th1, L ECTU R E   T Cells at Work    67 Th2, or Th17 This sets up a positive feedback loop which results in even more of the “selected” Th cells being produced In addition to positive feedback, there is also negative feedback at work For example, IFN‐γ made by Th1 cells actually decreases the rate of proliferation of Th2 cells, so that fewer Th2 cells will be produced And one of the Th2 cytokines, IL‐10, acts to decrease the rate of proliferation of Th1 cells The result of all this positive and negative feedback is a large number of helper T cells which are strongly biased toward the production of a certain subset of cytokines There is an important point about helper T cell bias which I want to be sure you understand Cytokines have a very limited range They can travel only short distances in the body before they are captured by cellular receptors or are degraded Consequently, when we talk about helper T cells being biased toward secreting a certain cytokine profile, we are talking about something very local Clearly, you wouldn’t want every Th cell in your body to be of the Th1 type, because then you’d have no way to defend against a respiratory infection Conversely, you wouldn’t want to have only Th2 cells, because the IgA or IgE antibodies made in response to the Th2 cytokines would be useless if you get a bacterial infection in your big toe In fact, it is the local nature of cytokine signaling which gives the immune system the flexibility to simultaneously mount defenses against many different invaders that threaten different parts of the body It is also important to note that dendritic cells are members of the innate system team Consequently, the innate immune system not only informs the adaptive system when there is danger, it also “coaches” the adaptive system to insure that the appropriate weapons are sent to the right places DELAYED‐TYPE HYPERSENSITIVITY There is an example of “signal calling” by Th cells that I think you’ll find interesting It is termed delayed‐type hypersensitivity (DTH), and it was first observed by Robert Koch when he was studying tuberculosis back in the latter part of the nineteenth century Koch purified a protein, tuberculin, from the bacterium which causes tuberculosis, and used this protein to devise his famous “tuberculin skin test.” If you’ve had this test, you’ll recall that a nurse injected something under your skin, and told you to check that area in a few days If the spot where you were injected became red and swollen, you were instructed to come back in to see the doctor Here’s what that’s all about The “something” you were injected with was Koch’s tuberculin protein If you have active TB or have been infected with it in the past, your immune system will include memory, Th1‐type helper T cells that were made in response to the infection When the nurse injects the tuberculin protein, dendritic cells stationed beneath the skin take up the protein and present tuberculin peptides to these memory cells  –  and they are reactivated Now the fun begins, because these Th cells secrete IFN‐γ and TNF  –  Th1‐type cytokines that activate resident tissue macrophages near the site of injection, and help recruit neutrophils and additional macrophages to the area The result is a local inflammatory reaction with redness and swelling: the signal that your TB test is positive Of course, the reason you have to wait several days for the test to “develop” is that memory helper T cells must be reactivated, proliferate, and produce those all‐important cytokines that orchestrate the inflammatory reaction On the other hand, if you have never been exposed to the tuberculosis bacterium, you will have no memory helper T cells to reactivate Without the cytokines supplied by activated Th cells, there will be no inflammatory reaction to the tuberculin protein, and your skin test will be scored as negative What is interesting here is that delayed‐type hypersensitivity is both specific and non‐specific The specificity comes from Th cells that direct the immune response after recognizing the tuberculin peptide presented by dendritic cells The non‐specific part of the reaction involves the neutrophils and macrophages that are recruited and activated by cytokines secreted by the Th cells This is yet another example of the cooperation that goes on between the adaptive and innate immune systems You may be wondering why the tuberculin used for the test doesn’t activate naive T cells, so that the next time you are tested, you will get a positive reaction The reason is that the tuberculin protein does not, by itself, cause an inflammatory reaction (i.e., a battle situation), and you remember that dendritic cells only mature and carry antigen to a lymph node if a battle is on Consequently, if a protein that is injected under your skin is judged by the innate system not to be dangerous, the adaptive immune system will not be activated This 68  LECT URE 6  T Cells at Work illustrates again how important the innate immune system is for initiating an immune response: If your innate system does not recognize an invader as dangerous and put up a fight, your adaptive system usually will just ignore the intrusion HOW CTLs KILL So far in this lecture, we have discussed what activated helper T cells Now it is time to focus on killer T cells Once a CTL has been activated, it proliferates rapidly to build up its numbers These effector T cells then leave the lymph node, enter the blood, and travel to the area of the body where the invaders they can kill are located When an effector T cell reaches the battle site, it exits the blood, and begins to hack away at infected cells Most killing by CTLs requires contact between the CTL and its target cell, and CTLs have several weapons they can use during this “hand‐to‐hand” combat One weapon CTLs employ involves the production of a protein called perforin Perforin is a close relative of the C9 complement protein that is part of the membrane attack complex Like its cousin, perforin can bind to cell membranes and drill holes in them For this to happen, a killer T cell’s TCRs must first identify the target Then adhesion molecules on the CTL hold the target cell close while the killer cell delivers a mixture of perforin and an enzyme called granzyme B onto the surface of the target cell Perforin damages the cell’s outer membrane, and when the cell tries to repair this damage, both granzyme B and perforin are taken into the cell in a vesicle made from the target cell’s membrane Once inside the target cell, the perforin molecules make holes in the entry vesicle, allowing the granzyme B to escape into the cytoplasm of the cell So perforin helps a CTL deliver granzyme B into the cytoplasm of its target cell There, granzyme B triggers an enzymatic chain reaction which causes the cell to commit suicide by apoptosis This kind of “assisted suicide” usually involves the destruction of the target cell’s DNA by the cell’s own enzymes One important feature of this type of killing is that it is “directed”: The CTL delivers its lethal cargo right onto the target cell, so that other cells in the area are not damaged during the slaughter After a killer T cell has made contact with its target, it only takes about half an hour to kill the cell, and during each attack, the CTL only uses a fraction of its perforin and granzyme B Consequently, a single killer T cell can execute multiple target cells The second way a CTL can kill is by using a protein on its surface called Fas ligand (FasL) which can bind to the Fas protein on the surface of a target cell When this happens, a suicide program is set in motion within the target cell, and, again, the cell dies by apoptosis Interestingly, natural killer cells use these same two mechanisms (perforin/granzyme B or FasL) to kill their targets It is worth mentioning here that there actually are two different ways a cell can die: by necrosis or by apoptosis Although the end result is the same (a dead cell), the two processes are quite different Cells usually die by necrosis either as the result of a wound (e.g., a cut or a burn) or when they are killed by an attacking virus or bacterium During necrosis, enzymes and chemicals that normally are safely contained within a living cell are released by the dying cell into the surrounding tissues, where they can real damage In contrast, death by apoptosis is much tidier As a cell dies by apoptosis, its contents are enclosed in little “garbage bags” (vesicles) made from the outer membrane of the dying cell These vesicles are then eaten and destroyed by nearby macrophages as part of their garbage collecting duty Consequently, during apoptosis, the contents of the target cell don’t get out into the tissues to cause damage So by killing their targets by inducing apoptosis rather than necrosis, CTLs can rid the body of virus‐infected cells without causing the collateral tissue damage that would result from necrotic cell death There is another reason why triggering cells to die by apoptosis is an especially effective way for killer T cells to destroy virus‐infected cells When virus‐infected cells die by apoptosis, the DNA of unassembled viruses is destroyed along with the target cell’s DNA In addition, DNA or RNA viruses that have reached various stages of assembly within the cell are enclosed in apoptotic vesicles and are disposed of by macrophages It is this ability to destroy infected cells and the viruses they contain by inducing apoptosis that makes a killer T cell such a potent antiviral weapon L ECTU R E   T Cells at Work    69 REVIEW In your body, dendritic antigen presenting cells are stationed beneath all surfaces that are exposed to the outside world By virtue of this geographic location, they can observe an invasion first hand In fact, the intelligence they acquire at the scene of the battle is complete enough to allow them to formulate a plan of action for the rest of the immune system This information is gathered in part through the dendritic cell’s pattern‐recognition receptors, which detect the “signatures” of different types of invaders Dendritic cells also have receptors which sense the cytokines given off by other immune system cells that are engaged in the battle In addition, non‐immune cells that reside in different areas of the body give off cytokines that imprint the dendritic cell with a regional identity – so it “remembers” where the battle is taking place Armed with all this information on the type of invader and the location of the attack, dendritic cells travel to nearby lymph nodes, where they activate T cells During this process, the game plan is conveyed to helper T cells in the form of co‐stimulatory molecules and cytokines expressed by the dendritic cells This information tells helper T cells which cytokines to make in order to orchestrate the appropriate defense against a particular invader In a sense, the dendritic cell functions as the coach of the immune system team, while the Th cell performs the duties of quarterback, calling the plays designed by the coach The dendritic cell is part of the innate immune system Consequently, the innate system not only determines when the adaptive system should be activated in response to danger, but it also instructs the adaptive system on which weapons to deploy and where to send them In response to the instructions delivered by dendritic cells, helper T cells produce combinations of cytokines that mobilize the weapons especially suited to deal with the invader that is attacking at the moment Although there are many different cytokines a given Th cell can secrete, the best studied combinations are called Th1, Th2, and Th17 Th1 cytokines are especially good at organizing the immune defense against viruses and bacteria which infect human cells Th2 helper T cells produce a combination of cytokines that is just right for defending against parasites or against bacteria that have breached the intestinal barrier And Th17 helper T cells secrete cytokines that mobilize the immune system to defend against fungi or extracellular bacteria Uncommitted Th cells also can be dispatched to the scene of the conflict where, under the influence of battle cytokines, they become committed to secreting a particular cytokine profile And once a Th cytokine profile has been established, positive and negative feedback tend to lock in this particular profile Importantly, the cytokines produced by helper T cells have a very short range, so their effects are quite “local.” This feature allows the immune system to defend against different types of invaders which attack different parts of the body When we are attacked by viruses or bacteria that infect human cells, dendritic cells can activate killer T cells and dispatch them to the area of the body under attack Killer T cells destroy infected cells by forcing them to commit suicide – a process called apoptosis When a cell dies by apoptosis, the contents of the cell are enclosed in vesicles which are quickly ingested by nearby macrophages This garbage disposal system keeps the potentially destructive chemicals and enzymes of the dying cell from getting out into the tissues and doing damage And it has the great advantage that the pathogens which infected the cell also are packaged up and disposed of SUMMARY FIGURE Here is our final summary figure, showing both the innate and adaptive systems – and the network they form Can you identify all the players, and you understand how they interact with each other? 70  LECT URE 6  T Cells at Work "KILL" Virus Membrane Attack Complex Virus " PS "O "O C3b PS ON I NATURAL KILLER CELL ,β N-α IF ZE IL-1 " TNF " " -γ IFN LL KI Virus INFECTED pDC Membrane Attack Complex ZE I ON B A C T E R I U M "KILL" COMPLEMENT ACTIVATED NK CELL α,β N- IF ILL " B A C T E R I U M C3b S LP "K IFN-γ "E ACTIVATED MACROPHAGE " "OPSONIZE" ACTIVATED B CELL WITH ANTIBODIES VIRGIN B CELL CD40 BCR CD40L VIRGIN HELPER T CELL (Th) B7 CD28 TCR MHC II TCR MHC I ACTIVATED IL-2 DR ITI C CE LL VIRGIN KILLER T CELL (CTL) N DE us Vir ACTIVATED Th ACTIVATED TN F N DE L ACTIVATED CTL CEL CD28 TCR IT IC B7 Antigen C3b O CR MHC II IVATED MA WITH ANTIBODIES ACT "OPSONIZE" Virus B A C T E R I U M "EA T" PH AG E "KILL" T" "EA CELL "KILL MACROPHAGE "E AT " AT " VIRUSINFECTED CELL DR ITI C Virus ND DE R L ECTU R E   T Cells at Work    71 THOUGHT QUESTIONS How does a helper T cell know which cytokine profile to produce? How does a helper T cell “call the plays” for B cells? How does a helper T cell orchestrate the actions of innate system players like macrophages and NK cells? Cytokines have a limited range Why is this a good thing? What is the difference between death by necrosis and death by apoptosis? ... language, how the immune system fits together – a book that presents the big picture of the immune system, without the jargon and the details How the Immune System Works is written in the form... attack by the same invader Lecture 11 The Intestinal Immune System, 10 3 The human intestines are home to trillions of bacteria, viruses, fungi, and parasites How the immune system deals with these... 978 -1- 118 -99777-2 (pbk.)   I Title.    [DNLM: 1.   Immune System physiology 2.  Immune System anatomy & histology 3.  Immune System physiopathology 4.  Immunity physiology.  QW 504]   QR1 81   616 .07’9
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